Vacuum Hold Down - Yay or Nay?

[progressiverain]

Hey guys,
Although I have been cruising these forums since before I got my shopbot 3-4 years ago I believe this will be my first actual post.
I've always thrown around the idea of a vacuum table but A: I was never really sure it would work for me B: I couldn't justify the brain damage and C: I couldn't justify throwing loads of money at this if I wasn't sure it work. Here we cut roughly 75% mdf and 25% acrylic. For the MDF we just screw through it into the spoil board and it's worked fine so far. The acrylic however we have to use very expensive hold down tape and spending $300 a month on that is getting a little old.
My main problem is that were always cutting through the material and leaving a skin is out of the question. On a single sheet we could have 25-50 pieces and that is way to much routing to even make it worth it. Also, our pieces can be pretty small too.

SO, what Im really wondering is if a vacuum table would even work for me. I can spend between 5-8k on one if needed.
Below is a link to an example file of what I would be cutting out of 1/2" MDF with two passes with a 1/4" double flute straight bit.

Oh, and I've got a 4x8 PRS Standard


http://www.5280events.net/mdf.crv


Any help or ideas would greatly appreciated!

[jerry_stanek]

It looks like a vac will hold those pieces ok. I have a four zone vac on my PRS Standard using 2 Fein vacs and they work real good. What do your parts for acrylic look like. You could always use the vac and a pressure foot I do this with small parts. Check out widgetworks for the foot. Also a down cut helps. I cut 1 inch buttons with mine using the pressure foot and only lost 2 out of 150.

[progressiverain]

It looks like a vac will hold those pieces ok. I have a four zone vac on my PRS Standard using 2 Fein vacs and they work real good. What do your parts for acrylic look like. You could always use the vac and a pressure foot I do this with small parts. Check out widgetworks for the foot. Also a down cut helps. I cut 1 inch buttons with mine using the pressure foot and only lost 2 out of 150.

The acrylic parts range. Some of them are pretty big which I know would be fine but sometimes Im cutting an entire sheet of 5" strips ranging in length from 24" to 96"
Im looking at the pressure foot right now. That looks very interesting.
I've tried the down cuts, for the mdf anyways, and it works but I burn through them so fast and it's almost impossible to find people who can resharpen them.

One thing Im still pretty confused about as far as the actual table goes. When you look at this picture:
http://www.woodweb.com/knowledge_base_images/zp/shop_built_cnc_vacuum_table_1.jpg
That isn't the spoil board is it? Don't you put a piece of wood ontop of that like LDF and it sucks through? Or am I way off base?

[curtiss]

If you are spending $300 a month on tape, you might be due for a change. I would suppose there is another vac table in your area to look at, seems like you might just need one zone if you are setting up full sheets most of the time.

[progressiverain]

Im so freakin confused. I've been reading past posts on pumps and there really seems to be no definitive answer as to what someone should use. Vane? Regen? Feins? I just want to be able to cut full sheets and not have to use 500 screws or $20 of tape.
99% of the time I cut full sheets and cut all the way through the material. Any time I don't use the full bed I would just use tape.
Cost really isn't a huge issue. Im not poor but I don't wanting to be spending $20,000 on pump either.

[myxpykalix]

Brady is the guru on this stuff, i couldn't see your file but if you say you cut small parts then it might be more advantageous to have smaller zones using pods that you can daisy chain and addon but thats just an opinion i'd defer to the experts.

If you wanted to make a whole table vacumn system, if you are going to use say four zones, pvc pipe and say 2 feins then I think i spent about $800.

But for me and the things i cut frankly i rarely use my vacumn setup. I think partly because i'm cheap. My reasoning is why run 2 feins for 6-8 hours for a hold down on a 3d carving when i can attach it to a jig and clamp it down. Plus i feel more secure that its not going to move.:eek:


after rereading, you say you cut whole sheets i can tell you that is when i use my vacumn because to hold down a whole sheet it works pretty good. I have found that depending on the bit you use the shaving stay in the kerf and help hold it from moving which can be good or bad...?

[jerry_stanek]

You say you won't leave a skin but what about tabs I use them some times then I use a 3/16 inch bearing bit in my trim router to cut them out.

[Brady Watson]

Brian,
If you have 3 phase in your shop, ShopBot sells a few really nice production duty pumps complete with EVERYTHING you need to build a multi-zone vacuum system except for the ULMDF bleeder board. It's worth looking into. Also, if you had another $1k over budget, they sell a Becker VTLF pump, which is the Rolls Royce of pumps. It sucks hard enough to keep Superman's cape on the table.

Call ShopBot and ask for Chris.

-B

[knight_toolworks]

depending on the size of the strips and the size of the bit you use the foot may nor may not work. the foot works with smaller bits the best like a 1/8" bit and lighter passes. you could do the bulk of the work with a larger bit then use a 1/8" bit and the foot for a final pass and cut them loose.

[dana_swift]

My nickels worth on the subject:

You can start cheap: Go with Feins. See how it works. Get some experience to guide your next step. You may quickly decide that is enough for you, or you just need a bigger pump or whatever. You may decide tape was cheaper.

I recommend you design a vacuum table that can be placed on top of the spoilboard and removed easily. (Thats what I did, but my vacuum is for a small table buddy.) Then you have the best of both worlds. Not only that but you can convert either way with little impact on production. This method will cost you about 2.5" of Z for the vacuum insert.

I went through several A/C vacuum pumps with dedicated gasket fixtures, before I finally spent the money on a Becker vacuum pump. Brady's description is right on. Its really sucks. HARD, you dont quite get as much CFM as with a blower, but much higher vacuums with enough CFM for me. I read on the forum where other vacuum systems get UP to 15" of mercury, when mine gets DOWN to that value I am worried parts may not stay stuck down. Thats when I am using tape- to cover the leaks.

Bad news, if you cut through with a vacuum, you will still be running around with tape covering the cuts to stop the leaks. At least you can use the cheapest tape you can get your hands on. But you may end up needing MORE tape than before.

Also if you are spending $20 a month on tape, that may be much cheaper than the electric bill on a several horsepower motor running all month.

I like vacuum a lot, but I love having the option of a conventional spoilboard OR vacuum.

Good luck!

[jerry_stanek]

Once you have the vac working you will wonder how you lived without it for so long. I have mine divided into 4 zones. My spoil board is glued down and divided by a strip of sintra. If I have to I can screw or use double sided tape also.

[kevin]

It seems you do these as a living

Its stupid cheap not to have one .I also can't imagine running the bot with out one

[Brady Watson]

In my early years I thought vacuum was all a bunch of foolishness that would never work...boy was I wrong & I sure did learn a lot along my journey from shopvacs to professional pumps. As I get older I am learning to pick what I need & want from the get go because there seems to be less & less time...plus there's always enough time & money to do it right the 2nd time! :D

-B

[progressiverain]

Thanks for all of your input guys.
You're right...I use this machine for a living and I am also paying my guys to deal with the tape and screws. It sounds pretty much like I need to go with the large industrial pumps if I really want consistently good results.

[gundog]

My .02 use vacuum and cut through the large parts leave a skin on the small parts and trim them out with a laminate trimmer. Don't limit yourself to just one method. I even screw parts to a piece of plywood at times and hold the plywood with my vacuum table. I think once you have vacuum you will save a ton of time I think trimming with a laminate trimmer is faster than tape or screws.

By the way I built a very functional vacuum table for my machine using house vac motors, ABS pipe & RV type knife blade valves for about $500 - $600 and it works great for my use. The vacuum table is a weekend job to complete. I cut a lot of plastic and some of the parts are under 1" and 1/2" thick I just leave a .050" skin and trim them out with a laminate trimmer.

Mike